There is lots of good advice so far. On one vs two subs, the trade offs are:
With one more expensive sub you can get something that is going to reproduce the really low movie effects (below 20Hz) better. The cost is less smooth in room response. You may be able to find a good sub placement for the main seating position, but the response will be less even for other seating positions.
With two lesser price subs, you will give up something on the bottom end, but you will get a more even response across all listening positions.
If its just you, buy the biggest sub you can afford and enjoy subwoofer nirvana from your throne. If you have other people watching with you AND they actually care about low frequency stuff in movies, consider two smaller subs.
One final thought. I didn't care much about earth shattering rumble before I got a sub. In fact, all I cared about was that I wouldn't miss anything in music. Once I started watching movies with my new setup I developed a new appreciation for movie LFE.
